1. Technical Field
The present invention relates to an ultrasonic imaging apparatus which extracts images of the interior of a subject's body using ultrasound, to an ultrasonic probe used in the ultrasonic imaging apparatus, and to an ultrasonic imaging method using the ultrasonic imaging apparatus.
2. Background Art
Minimally invasive therapy is a technique in which surgery is performed without requiring a large incision. Consequently, it has significant merits for the patient including improving QOL (quality of life) after surgery, lessening the risk of infection due to surgery, and shortening the length of stay in hospital. However, if the surgeon is unable to adequately grasp the conditions of the site of operation, the prerequisites for minimal invasion are significantly eroded. Thus, image guidance can be considered as an indispensable part for minimally invasive therapy. In particular, in the case of surgery involving the circulatory organ, since shape variations over time are remarkable depending on the technique employed, in addition to an image taken by an endoscope or the like that provides a localized field of view, guidance by an image showing the overall constitution including the organ of interest and the periphery thereof is necessary. For example, as disclosed in JP Patent Publication (unexamined application) No. 4-312446, in MRI (magnetic resonance imaging), an open-type MRI apparatus which allows a surgeon access to the inside of an MRI gantry from outside has been practically applied. Therefore, MRI can be considered as an appropriate means for the above-mentioned image guidance means.
Further, in recent years ultrasonic imaging apparatus have been provided which offer high speed, high definition, and enhanced functionality, in addition to their interactivity. Thus, ultrasonic apparatus can augment an optical endoscope by providing the surgeon with an additional means of visual inspection, as well as augmenting MRI with its real time property with regard to the localized image of the site of operation. With regard to the imaging region also, while MRI involves the entire body, ultrasound involves only the local region. Therefore, as image guidance for minimally invasive therapy, provision of the two imaging means of MRI and ultrasound, whose relationship is such that they compensate for each other, is extremely useful.
In order to obtain a high definition image ultrasonic imaging, it is known that it is desirable that the transmit pulse be short in a time-base direction, that is, that it be broadband in frequency space. In particular, accompanying the evolution of second harmonic imaging technology, the necessity of an ultrasonic probe capable of transmitting a broadband ultrasonic pulse is well established. As a means for the realization thereof, a structure in which an acoustic matching layer is overlaid on a transmitting side of an ultrasonic transducer comprising PZT (lead zirconate titanate) ceramic, and backing material that attenuates an ultrasonic wave is provided on the back thereof is known. The layer on the PZT side of the acoustic matching layer comprising a plurality of layers and the backing material comprise a mixture of polymer material and metal powder, and are used after adjusting the acoustic impedance thereof to a suitable value. The acoustic impedance is an amount indicated by the product of the acoustic velocity and density of the material, and reflection of sound occurs at a surface between two materials whose acoustic impedance are different. As the acoustic impedance of PZT is about 34×106 kg/m2s and the acoustic impedance of a subject is about 1.5×106 kg/m2s, as shown in FIG. 1, an intermediate acoustic impedance material is inserted between the subject and the PZT, and acoustic impedance is gradually altered along the propagation direction of the sound, thereby suppressing unnecessary reflection. Regarding the backing material, by employing a material with a large attenuation factor, such as rubber, as a base, the probe can be made such that sound entering the backing material from the PZT is not reflected from the end of the opposite side and returned, which is the acoustic role of the backing material.
However, there are a further two problems to be solved in taking an MRI image when an ultrasonic probe is placed inside an MRI gantry. Specifically, it is necessary to suppress the influence imparted by the material of the ultrasonic probe itself to the surrounding static magnetic field as well as the influence on MRI image of leakage electromagnetic waves generated by the electric signal sent to the ultrasonic probe. With regard to one part of the acoustic matching layer and the backing material, in our studies up to now we considered that selecting material that is nonmagnetic would serve as a suitable countermeasure, however we found that if even the slightest magnetic impurity is included in the material the MRI image was greatly distorted by the influence thereof.
An object of the present invention is, firstly, to control magnetic impurity in the ultrasonic probe material and provide an ultrasonic probe in which the ultrasonic probe material itself does not impart an influence to an MRI image.
The second problem, that is the influence on the MRI image by leakage electromagnetic waves from the ultrasonic probe, resulting from the ultrasonic probe being connected to the ultrasonic imaging apparatus, will be described. During transmitting of ultrasonic waves from the ultrasonic probe to the subject, leakage electromagnetic waves from the probe are strong, and it is thus difficult to perform ultrasound imaging and MRI imaging at the same time. Therefore, as a countermeasure, it is necessary to take ultrasound and MRI images at slightly different points in time.
However, to perform such quasi-simultaneous observation, it is not enough simply that the material of the ultrasonic probe does not impart an influence to the MRI image. Specifically, if the power of the ultrasonic imaging apparatus is turned off during imaging by MRI it is only a problem of the material of the probe. But because current ultrasonic imaging apparatus are controlled by computers, turning the power on and off requires time. Thus, when performing quasi-simultaneous observation it is not realistic to turn off the power of the ultrasonic imaging apparatus each time imaging is to be performed by MRI.
Therefore, another object of the present invention is to provide an ultrasonic imaging apparatus in which leakage electromagnetic waves are not generated from an ultrasonic probe during imaging by MRI. A further object of the present invention is to provide a novel method of quasi-simultaneous observation by MRI and an ultrasonic imaging apparatus.